1. Field of the Invention
This invention relates to an improved catalyst system for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petro-chemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. The catalytic reforming unit therefore must operate more efficiently at higher severity in order to meet these increasing aromatics and gasoline-octane needs. This trend creates a need for more effective reforming processes and catalysts.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. The increased sensitivity of these selective catalysts to sulfur in the feed also is known. It is believed that the extreme, unanticipated sulfur sensitivity of these reforming catalysts is primarily responsible for the lengthy development period and slow commercialization of this dehydrocyclization technology. Unit operations and processing costs would benefit from novel methods for sulfur management such as the process of the present invention.
3. Related Art
The desulfurization of naphtha feedstocks to a reforming process using sulfur sorbents is widely disclosed. U.S. Pat. Nos. 4,225,417 and 4,329,220 (Nelson) teach a reforming process in which sulfur is removed from a reforming feedstock using a manganese-containing composition. Preferably, the feed is hydrotreated and the sulfur content is reduced by the invention to below 0.1 ppm. U.S. Pat. Nos. 4,534,943 and 4,575,415 (Novak et al.) teach an apparatus and method, respectively, for removing residual sulfur from reformer feed using parallel absorbers for continuous operation; ideally, sulfur is removed to a level of below 0.1 ppm. U.S. Pat. No. B1 4,456,527 (Buss et al.) discloses the reforming of a hydrocarbon feed having a sulfur content of as low as 50 ppb (parts per billion) with a catalyst comprising a large-pore zeolite and Group VIII metal. A broad range of sulfur-removal options is disclosed to reduce the sulfur content of the hydrocarbon feed to below 500 ppb. U.S. Pat. No. 4,741,819 (Robinson et al.) is drawn to a method for removing residual sulfur from a hydrotreated naphtha feedstock comprising contacting the feedstock with a less-sulfur sensitive reforming catalyst, a sulfur sorbent, and a highly selective reforming catalyst. U.S. Pat. No. 4,831,206 (Zarchy) discloses a hydrocarbon conversion process comprising sulfur conversion, liquid-phase H.sub.2 S removal with zeolite, and vaporization of the product to the reaction zone. A platinum/L-zeolite catalyst having improved sulfur tolerance through the incorporation of rhenium is revealed in U.S. Pat. No. 4,954,245 (Miller et al.) None of the above references anticipate or suggest a catalyst system comprising a physical mixture of a conversion catalyst and a sulfur sorbent, however.
Other applications of mixed-catalyst systems, in which there is synergy in having the features of each catalyst in proximity, are known. U.S. Pat. No. 3,764,520 (Kimberlin et al.) and U.S. Pat. No. 3,769,202 (Plank et al.) disclose hydrocarbon-conversion processes, specifically catalytic cracking, using catalyst mixtures of two zeolites of differing pore size. A process using a physical mixture of a reforming catalyst and a hydrocracking catalyst is disclosed in U.S. Pat. No. 4,212,727 (Antos). Removal of nitrogen oxides from gas streams using a physical mixture of a copper catalyst and a catalyst containing a combination of metals is taught in U.S. Pat. No. 4,257,918 (Ginger). U.S. Pat. No. 4,418,006 (Kim et al.) discloses a catalyst system comprising a physical particle-form mixture of a noble-metal catalyst free of zeolite and a zeolite free of metal. U.S. Pat. No. 5,059,304 (Field) teaches a combination of desulfurization with a platinum on alumina catalyst to avoid significant cracking and a sorbent comprising a supported Group I-A or II-A metal; the catalyst and sorbent may be intermixed. Nevertheless, none of these references discloses or suggests a mixture of a sulfur sorbent with a sulfur-sensitive conversion catalyst.